What makes a fertilizer fire so volatile

A fire anywhere is cause for concern, but a fire at a fertilizer plant is a potential catastrophe.

That's because ammonium nitrate, a chemical commonly used in agricultural fertilizers, is a highly explosive compound, as shown by the massive fireball at the fertilizer plant in the town of West, Texas, on Wednesday.

Nitrogen, phosphorous and potassium are essential plant nutrients, and fertilizers are graded by the amounts of these elements the fertilizers contain, also called their "NPK rating" (from those elements' abbreviations on the periodic table).

Ammonium nitrate, or NH4-NO3, is frequently added to improve a fertilizer's nitrogen content. It's relatively stable under most conditions and is inexpensive to manufacture, according to Slate, making the chemical a popular alternative to other, more expensive nitrogen sources.

But ammonium nitrate has a potentially lethal downside: If it comes into contact with an open flame or other ignition source, it explodes violently. The explosive force occurs when solid ammonium nitrate decomposes very rapidly into two gases, nitrous oxide and water vapor.

When the chemical exploded, it caused a blast powerful enough to knock people to the ground in Galveston, Texas, 10 miles (16 kilometers) away.

The detonation also caused a chain reaction when a nearby ship, also carrying ammonium nitrate, exploded, setting fires at chemical tanks and oil refineries near the port. An estimated 581 people were killed in the disaster.

But not all disasters involving ammonium nitrate are accidents: The fertilizer was packed into a rented truck and used by terrorists Timothy McVeigh and Terry Nichols to kill 168 people in the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City.

The chemical was used again in the 2002 nightclub bombing in Bali that killed 202 people, in the 2011 Oslo bombing by Anders Behring Breivik, which killed eight people, and in numerous other terrorist attacks.

Because of its danger and potential use by terrorists, ammonium nitrate is subject to strict regulation in most places. In 2011, according to NBC News, the Department of Homeland Security established rules limiting the sale of the compound, which is also used as an explosive in the construction and mining industries.